1. Field of the Invention
The present invention relates, in general, to spacer grids used for supporting fuel rods in nuclear fuel assemblies and, more particularly, to a spacer grid provided with side weld supports and flow mixing vanes at the intersections of its inner straps for creating a lateral flow of coolant within coolant channels, in addition to improving the structural strength of the spacer grid and the integrity of the nuclear fuel assembles.
2. Description of the Prior Art
As shown in FIG. 1, a conventional nuclear fuel assembly 500 of a nuclear reactor comprises a plurality of fuel rods 502, each fabricated such that a fissionable fuel material, such as a uranium pellet, is contained in a hermetically sealed elongated tube, known as the cladding. The fuel rods 502 are supported within the fuel assembly by a plurality of spacer grids 501, which also generate a strong turbulent flow of coolant within the fuel assembly. The bottom of the fuel assembly is covered with a bottom end piece 503, which stably connects the fuel rods 502 to the lower structure of a reactor core. The top of the fuel assembly is covered with a top end piece 504, which stably connects the fuel rods 502 to the upper structure of the reactor core. In the fuel assembly, the spacer grids 501 and the two end pieces 503 and 504 are integrated into a single structure using a plurality of guide tubes 505. The guide tubes 505 also form a plurality of passages for receiving a variety of monitoring tubes used for measuring the operational conditions of the reactor.
As shown in FIG. 2, each spacer grid 501 is produced by intersecting a plurality of thin metal inner straps at right angles to form an egg-crate pattern, and spot-welding the interlaced straps at the top and bottom of their intersections, and so a desired integrity of the spacer grid 501 is accomplished. A plurality of flow mixing vanes 540 are provided on the top of each intersection of the spacer grid 501 for changing a portion of axial flow coolant 510 to cross flow 520 or swirl flow 530. The spacer grid 501, fabricated by the interlaced inner straps, defines a plurality of four-walled cells for receiving and holding the fuel rods 502 therein as shown in FIG. 3. In each of the cells, a plurality of grid springs 560 and a plurality of strong dimples 570 are formed on the inner straps such that the springs 560 and dimples 570 face each other. The springs 560 and dimples 570 support the fuel rods 502 in the spacer grids 501.
In the fuel assembly, the fuel rods 502 are axially set in the cells of the spacer grids 501 such that four fuel rods 502 inside four adjacent cells of each spacer grid 501 form a coolant channel 550xe2x80x2 as shown in FIG. 4, and so the coolant axially flows along the channel 550xe2x80x2. The coolant channel 550xe2x80x2 is open at each side thereof by a gap formed between two fuel rods 502, and has an intersection of the inner straps at the center thereof. The fuel rods of a nuclear fuel assembly typically have different thermal outputs due to an imbalance in the neutron flux distribution, and so the coolant flowing through some coolant channels surrounded by fuel rods having high thermal outputs is highly increased in its temperature in comparison with the coolant flowing through other coolant channels surrounded by fuel rods having low thermal outputs. In order to improve thermal efficiency of a reactor core, it is necessary to accomplish a uniform coolant temperature distribution within the coolant channels. The above objective may be accomplished by a provision of flow mixing vanes on the top of the spacer grid. That is, the flow mixing vanes accomplish a uniform coolant temperature distribution and prevent a part of the coolant from being overheated during an operation of a nuclear reactor.
When the coolant passes through the spacer grids, frictional and form pressure losses are presence due to the friction between the coolant and surface of the grid and the momentum exchange as area change of the flow path, respectively.
Such pressure losses result in an energy loss, which requires high capacity of main coolant pump to compensate the losses. Therefore, it is preferred to design the nuclear reactor system with low pressure losses.
The nuclear fuel assembly is fabricated by mounting the upper and lower end pieces to the bundle formed by inserting the fuel rods in the spacer grids. On inserting in the spacer grids, the fuel rods may contact with the vanes and cause the vane deformation, which may scratch the external surfaces of the fuel rods. Therefore, in the prior art, the flow mixing vanes of the spacer grid have been designed such that they do not interfere with the fuel rods.
Conventional flow mixing vanes for spacer grids may be referred to U.S. Pat. No. 4,692,302 (Inventors: Edmund E. Demario et al., Applicant: Westinghouse Co. Ltd.) and U.S. Pat. No. 5,440,599 (Inventors: Thomas Rodack et al., Applicant: Combustion Engineering Co. Ltd.).
In a flow mixing device disclosed in U.S. Pat. No. 4,692,302, two flow mixing vanes are formed at each intersection of the inner straps of a spacer grid such that the two vanes are formed along the top edge of one of two inner straps crossing each other at the intersection and are deflected in opposite directions. This flow mixing device changes a portion of coolant along the central axis of each channel to a cross flow guided to the gaps between fuel rods.
In the above flow mixing device, a weld window 580 and a weld tap 590 are provided at the gap between two flow mixing vanes 540 as shown in FIGS. 5 and 6.
The interlaced straps are welded at the upper and lower intersection and then the welding taps 590 are changed into weld beads 590xe2x80x2. The weld beads 590xe2x80x2 increase hydraulic resistance by generating flow separation downstream of the beads thereof, thus reducing the flow mixing efficiency of the vanes 540. In addition, the weld window 580 positioned between two vanes 540 makes the edges of the two vanes become closer to the rod, thus may allowing an interference of the vanes 540 with the fuel rods.
In a flow mixing device disclosed in U.S. Pat. No. 5,440,599, a triangular vane support extends upward from the top edge of one of two inner straps crossing each other at each intersection, with two mixing vanes formed on opposite sides of the triangular vane support and deflected in opposite directions.
This flow mixing device changes a portion of coolant along the central axis of the channel to the gaps between the fuel rods. However, this flow mixing device is problematic in that a weld tap is provided under the vane support and becomes a weld bead when welding the interlaced inner straps together.
The weld beads 590xe2x80x2 increase hydraulic resistance by generating flow separation downstream of the beads thereof, thus reducing the flow mixing efficiency of the vanes 540.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an objective of the present invention is to provide a spacer grid for nuclear fuel assemblies, which has a plurality of side weld supports, that is, main supports, upper sub-supports and lower sub-supports, on its interlaced inner straps, with two flow mixing vanes integrally extending upward from each of the main supports, and which is fabricated by seam-welding the interlaced inner straps to each other along the upper axial junction lines of the crossing main and upper sub-supports at the top of the intersections, and along the lower axial junction lines of the crossing lower sub-supports at the bottom of the intersections, and which prevents the flow mixing vanes from interfering with fuel rods during a fuel rod installation process, accomplishes a desired integrity, improves the coolant mixing efficiency of the flow mixing vanes, and reduce form pressure loss caused by weld beads.
In order to accomplish the above objects, the present invention provides a spacer grid used in a nuclear fuel assembly such that a plurality of spacer grids are regularly and transversely arranged along the fuel assembly to support a plurality of fuel rods within the fuel assembly while maintaining a desired pitch of the fuel rods, comprising:
A plurality of first inner straps each having a rectangular-shaped first strap body, with a plurality of main supports and a plurality of upper sub-supports alternately formed along a top edge of the first strap body while being spaced apart from each other at regular intervals, a plurality of lower sub-supports formed along a bottom edge of the first strap body while being spaced apart from each other at regular interval, a pair of flow mixing vanes symmetrically formed along a top edge of each of said main supports, and a plurality of upper vertical slits extending from center of top edge of each said main supports and each upper sub-supports of the first strap body toward a middle of said first strap body; and
A plurality of second inner straps each having a rectangular-shaped second strap body, with a plurality of main supports and a plurality of upper sub-supports alternately formed along a top edge of the second strap body while being spaced apart from each other at regular intervals, a plurality of lower sub-supports formed along a bottom edge of the second strap body while being spaced apart from each other at regular interval, a pair of flow mixing vanes symmetrically formed along a top edge of each of said main supports of the second strap body, and a plurality of lower vertical slits extending from center of bottom edge of each said lower sub-supports of the second strap body toward a middle of said second strap body;
whereby said first and second inner straps are interlaced at right angles at the vertical slits such that the interlaced inner straps form a plurality of square cells for receiving the fuel rods, with the main supports of the first and second inner strap crossing the upper sub-supports of the second and first inner straps while forming a plurality of upper axial junction lines, and the lower sub-supports of the first and second inner straps crossing each other while forming a plurality of lower axial junction lines, said inner straps being seam-welded to each other along the upper and lower junction lines to form a plurality of side weld lines, and said flow mixing vanes of the interlaced inner straps guiding axial flows of coolant to gaps between the fuel rods, thus forming cross flows of coolant.